A small pore area material (“SPM”) is a type of foam, which may be thought of as a dispersion of gas bubbles within a liquid, solid or gel (see IUPAC Compendium of Chemical Terminology (2d ed. 1997)). Specifically, and as used herein, an SPM is a foam having a density of less than about 1000 kilograms per cubic meter (kg/m3) and a small pore structure in which the average pore area is less than about 500 μm2. Average pore area, as used herein, is the average of the pore areas of at least the 20 largest pores identified by visual examination of images generated by scanning electron microscopy (“SEM”). These pore areas were then measured with the use of ImageJ software, available from NIH.
One type of SPM is a low density microcellular material (“LDMM”). Specifically, and as used herein, an LDMM is an SPM having a microcellular structure in which the average pore diameter is less than about 1000 nanometers (nm) which is determined by measuring the average pore area and then calculating the average pore diameter by using the formula: area=Πr2. For example, an average pore area of 0.8 μm2 corresponds to an average pore diameter of 1000 nm.
Certain LDMMs are known and have been used in a variety of applications including, but not limited to, thermal barriers and insulation, acoustical barriers and insulation, electrical and electronic components, shock and impact isolators, and chemical applications. See, e.g., Materials Research Society, vol. 15, no. 12 (December 1990); Lawrence Livermore National Labs Materials, Science Bulletin UCRL-TB-117598-37; U.S. Pat. No. 4,832,881. For a foam having an average pore diameter greater than about 300 nm, pore area is the preferable characterization of the pores as it can be more easily measured using, e.g., SEM images with available software that calculates pore and particle size.
The usefulness of any particular foam depends on certain properties, including, but not limited to, bulk density, bulk size, cell or pore structure, and/or strength. See, e.g., “Mechanical Structure-Property Relationship of Aerogels,” Journal of Non-Crystalline Solids, vol. 277, pp. 127–41 (2000); “Thermal and Electrical Conductivity of Monolithic Carbon Aerogels,” Journal of Applied Physics, vol. 73 (2), 15 Jan. 1993; “Organic Aerogels: Microstructural Dependence of Mechanical Properties in Compression,” Journal of Non-Crystalline Solids, vol. 125, pp. 67–75 (1990). For example, density affects, among other things, a foam's solid thermal conductivity, mechanical strength (elastic modulus), and sound velocity. In general, lowering the density of a foam will also lower its solid thermal conductivity, elastic modulus, and longitudinal sound velocity. However, a foam's density cannot be too low otherwise it will not satisfy the mechanical stability of its intended application.
In addition, a foam will generally be more useful and better suited to more applications if it can be produced in a variety of shapes and sizes. Further, pore structure affects, among other things, the gaseous thermal conductivity within a foam, as well as mechanical strength and surface area. In general, smaller pore size (average pore area and/or average pore diameter) improves a foam's physical properties in these areas if the density of the material does not increase. It is therefore desirable in most cases to lower density and pore size until a minimum is reached for both cases. This can be difficult to achieve since, in most materials, these properties counteract each other so that decreasing density leads to larger pore sizes.
Other important properties, at least for purposes of commercialization, include ease and flexibility of manufacture, for example, the ability to withstand the stresses that typically exist during manufacture that cause degradation (e.g., shrinkage and/or cracking), and the ability to make foams having a broad range of properties, sizes and shapes that can also be made in situ.
Generally, foams can be classified by their pore size distribution. Average pore diameter may fall within three ranges: (1) micropore, in which the average pore diameter is less than about 2 nm; (2) mesopore, in which the average pore diameter is between about 2 nm and about 50 nm; and (3) macropore, in which the average pore diameter is greater than about 50 nm. See IUPAC Compendium of Chemical Terminology (2d ed. 1997). An example of a foam having a micropore structure is a xerogel. An example of a foam having a mesopore structure, and a particularly useful foam, is an aerogel. Generally, an aerogel is a type of LDMM (and thus it is also an SPM) in which gas is dispersed in an amorphous solid composed of interconnected particles that form small, interconnected pores. The size of the particles and the pores typically range from about 1 to about 100 nm. Specifically, and as used herein, an aerogel is an LDMM (and thus it is also an SPM) in which: (1) the average pore diameter is between about 2 nm and about 50 nm, which is determined from the multipoint BJH (Barrett, Joyner and Halenda) adsorption curve of N2 over a range of relative pressures, typically 0.01–0.99 (“the BJH method” measures the average pore diameter of those pores having diameters between 1–300 nm and does not account for larger pores); and (2) at least 50% of its total pore volume comprises pores having a pore diameter of between 1–300 nm.
Another way to classify foams is by the number of closed or open pores they have. For example, closed pore foams have a high number of sealed or encapsulated pores that trap the dispersed gas such that the gas cannot easily escape. See, e.g., U.S. Pat. Nos. 6,121,337; 4,243,717; and 4,997,706. Open pore foams have a lower number of sealed or encapsulated pores and, as such, the interior spaces and surfaces are accessible and the gas within them may be evacuated. Thus, foams with more open pores are more desirable for evacuated thermal insulation, chemical and catalytic reactions, and electrical applications. For example, only open pore materials can be evacuated for increased thermal insulation commonly known as vacuum insulation, many chemical and catalytic reactions operate by accessing activated surfaces on the interior of foams thus more open spaces and surfaces increase reaction efficiencies, and many electrical applications also operate by accessing conducting surfaces thus more open surfaces increase electrical efficiencies. In general, the known SPM foams are open pore foams in which nearly all the pores are open. Other foams that are not SPMs typically have fewer open pores, in which generally less than about 80% of the pores are open.
SPM foams may be further classified, for example, by the type of components from which they are made. For example, inorganic aerogel foams may be made using silica, metal oxides or metal alkoxide materials and typically exhibit high surface area, low density, optical transparency and adequate thermal insulation properties. See, e.g., U.S. Pat. Nos. 5,795,557; 5,538,931; 5,851,947; 5,958,363. However, inorganic aerogels have several problems. For example, the precursor materials are relatively expensive, sensitive to moisture, and exhibit limited shelf-life. See, e.g., U.S. Pat. No. 5,525,643. Also, the processes used to make inorganic aerogels are typically expensive and time-consuming requiring multiple solvent-exchange steps, undesirable supercritical drying (discussed in more detail below) and/or expensive reagents for the modification of the gel surfaces. See, e.g., “Silica Aerogel Films Prepared at Ambient Pressure by Using Surface Derivatization to Induce Reversible Drying Shrinkage,” Nature, vol. 374, no. 30, pp. 439–43 (March 1995); “Mechanical Strengthening of TMOS-Based Alcogels by Aging in Silane Solutions,” Journal of Sol-Gel Science and Technology, vol. 3, pp. 199–204 (1994); “Synthesis of Monolithic Silica Gels by Hypercritical Solvent Evacuation,” Journal of Materials Science, vol. 19, pp. 1656–65 (1984); “Stress Development During Supercritical Drying,” Journal of Non-Crystalline Solids, vol. 145, pp. 3–40 (1992); and U.S. Pat. No. 2,680,696.
In contrast, organic SPM foams typically exhibit lower solid thermal conductivity and can be readily converted into low density, high surface area carbonized-foams that exhibit high electrical conductivity. Moreover, the precursor materials used to make organic SPMs tend to be inexpensive and exhibit long shelf-lives. See, e.g., “Aerogel Commercialization: Technology, Markets, and Costs,” Journal of Non-Crystalline Solids, vol. 186, pp. 372–79 (1995). Further, organic SPMs can be opaque (useful to reduce radiative thermal transfer) or transparent, although such opaque foams do not require opacification. As a result, generally, opaque organic SPMs are more desirable, especially for electronic applications and thermal applications in which optical transparency is not desired.
Foams, including SPM foams, can also be classified by their bulk properties. Monolithic foams, or monoliths, can be defined as being bulk materials having volumes greater than 0.125 mL, which corresponds to a block of material having a volume greater than 125 mm3 (i.e., 5 mm×5 mm×5 mm). Thin film and sheet foams can be defined as a coating, less than 5 mm thick, formed on a substrate. Granular or powder foams can be defined as comprising particle sizes of having volumes less than 0.125 mL. In general, foams that can be made in monolithic form have advantages over thin film or granular foams. For example, monolithic foams can be made for a wide variety of applications in which thin films, sheets or granulars would not be practical. For example, most thermal insulation, acoustical attenuation and kinetic (shock absorption) applications require thicker insulating material that cannot be provided by thin films or sheets. And, granular materials tend to settle and are not mechanically stable. Many chemical and catalytic applications also require more material than can be provided by thin films or sheets. Even some electrical applications require monolithic materials such as fuel cells and large capacitor electrodes.
In general, organic SPMs made using non-critical drying methods have been limited to LDMMs in thin film or granular forms. Organic, monolithic LDMMs generally have not been made using non-critical drying methods with one exception which took four days to prepare. See U.S. Pat. No. 5,945,084.
Further, although large monolithic inorganic aerogels have been made, such shapes and sizes have been limited and these inorganic aerogels have been made using undesirable supercritical drying methods (as explained below). For example, silica aerogels have been made in the following shapes and sizes: (1) a sheet 1 cm thick and having a length and width of 76 cm (corresponding to a volume of 5.776 liters); and (2) a cylinder 12 inches long having a diameter of 8 inches (corresponding to a volume of 9.884 liters).
Organic aerogels made using supercritical drying methods, however, have much more limited shapes and sizes, e.g.: (1) a sheet 1 inch thick and having a length and width of 12 inches (corresponding to a volume of 2.36 liters); and (2) a disk 3 inches thick having a diameter of 8 inches (corresponding to a volume of 2.47 liters). No organic monolithic aerogel is known whose smallest dimension is greater than 3 inches. Further, no organic monolithic aerogel is known that is made using non-critical drying techniques where the smallest diameter is greater than 5 mm. In addition, many of the known organic monolithic foams lack sufficient structural strength to withstand the stresses arising during manufacture. As a result, these foams tend to shrink and some also crack during manufacture.
In general, foams can be made using a wide variety of processes. See, e.g., U.S. Pat. Nos. 6,147,134; 5,889,071; 6,187,831;and 5,229,429. However, aerogels have been typically made using well known “sol-gel” processes. The term “sol” is used to indicate a dispersion of a solid in a liquid. The term “gel” is used to indicate a chemical system in which one component provides a sufficient structural network for rigidity, and other components fill the spaces between the structural units. The term “sol-gel” is used to indicate a capillary network formed by interlinked, dispersed solid particles of a sol, filled by a liquid component.
The preparation of foams by such known sol-gel processes generally involves two steps. In the first step, the precursor chemicals are mixed together and allowed to form a sol-gel under ambient conditions, or, more typically, at temperatures higher than ambient. In the second step, commonly referred to as the “drying step,” the liquid component of the sol-gel is removed. See, e.g., U.S. Pat. Nos. 4,610,863; 4,873,218; and 5,476,878. The ability to dry the sol-gel is in part dependent on the size of the foam. A larger foam will require more intensive drying because of the longer distance the solvent must pass from the interior of the foam to the exterior. A sol-gel that is dried in a mold or container will require that the liquid travel through the sol-gel to the open surface of the mold or container in order for the liquid component to be removed.
Conventional supercritical drying methods usually require the undesirable and potentially dangerous step of supercritical extraction of the solvent. In the case of direct supercritical extraction (a process wherein the solvent in which the sol-gel is formed is removed directly without exchanging it for another solvent), the solvent that is being extracted is most typically an alcohol (e.g., methanol), which requires high temperatures and pressures for extraction. Such conditions require the use of highly pressurized vessels. Subjecting alcohols to the high temperatures and pressures increases the risk of fire and/or explosion. Methanol poses the additional risk of toxicity.
Known sol-gel processes have several additional problems. In many instances, the precursor materials used are expensive and can be dangerous under the conditions used in conventional supercritical drying. Also, the resulting foams have been made in limited sizes and shapes due to constraints inherent in the known manufacturing processes and they also tend to exhibit cracking and/or shrinkage.
Another problem with conventional drying methods is that the drying step is time consuming and frequently quite tedious, typically requiring one or more solvent exchanges. See, e.g., U.S. Pat. Nos. 5,190,987; 5,420,168; 5,476,878; 5,556,892; 5,744,510; and 5,565,142. A further problem is that conventional drying methods sometimes require the additional step of chemically modifying the sol-gel. See, e.g., U.S. Pat. No. 5,565,142;“Silica Aerogel Films Prepared at Ambient Pressure by Using Surface Derivatization to Induce Reversible Drying Shrinkage,” Nature, vol. 374, no. 30, pp. 439–43 (March 1995).
For example, the most common process for aerogel production involves exchanging the solvent in which the sol-gel is formed (typically alcohol or water) with liquid carbon dioxide, which is then removed by supercritical extraction. Although the supercritical extraction of carbon dioxide requires relatively low temperatures (under 40° C.), it requires very high pressures (generally above 1070 psi). And, although carbon dioxide is non-flammable, the solvent-exchange step is very time consuming.
Moreover, even the known processes using ambient (non-critical) drying methods have deficiencies in that they do not produce low density monolithic foams, but rather thin films or granules.
As explained above, the known processes tend to produce organic aerogels having limited shapes and sizes. One reason for this is that the mold or container in which the foam is made is limited in size and/or shape. As a result, such processes do not allow for the extraction of foams where the distance the solvent must pass is very large.
An example of a known process for making foams is U.S. Pat. No. 5,565,142, which describes certain inorganic foams produced using evaporative drying methods. The described process requires solvent exchange and a further step wherein the sol-gel is chemically modified. Similarly, U.S. Pat. No. 5,945,084 describes the production of resorcinol foams by evaporative drying processes in which the lowest reported density of these foams is greater than 400 kg/M3. However, these foams exhibit relatively high thermal conductivity and require an excessive amount of time to gel, cure and dry. One example took more than four days to complete.
Although known foams may exhibit some of the above-described useful properties, no known foam exhibits all of these properties. Thus, an organic, small pore area, open cell foam that can have a wide variety of monolithic forms with sufficient structural strength and that optionally can be formed in situ is still needed.